The invention relates to a device and methods for coating substrates in a vacuum, wherein a plasma is to be generated from a target and ionized particles of the plasma are to be deposited on the substrate in the form of a layer, as have long been successfully used for a very wide range of known PVD processes.
In particular, the invention can be used to supplement the so-called laser arc method, in which an arc discharge in a vacuum can be ignited by means of a pulsed laser beam and the ionized flow of particles can be guided to a substrate together with the plasma obtained by means of the arc discharge and the ionized particles can be deposited on this substrate in the form of a layer. However, the invention may also be employed in a method which is known per se and in which an arc discharge in a vacuum is used to generate the plasma without the arc discharge being initiated by a laser beam. In this case, the arc discharge can in a known way either be ignited solely by a sufficiently high voltage between an anode and a target connected as cathode, and secondly it is possible to initiate the ignition by means of electrically conductive ignition elements as a result of short-circuiting.
A further possibility in which the solution according to the invention can be employed to good effect is the generation of a plasma on a target by area is restricted to approx. 150 mm on account of the powerful magnetic fields required and the electric power required for this purpose. The coating rate achieved by the methods is reduced to approx. 15-20% symbol compared to that without use of the filter.
Furthermore, JP 05-279844 A Patent Abstracts of Japan, Vol. 018, No. 068 (C-1161) describes a device for coating substrates in a vacuum. In this case, a plasma is generated by means of a laser beam which is directed onto a target. The electrically charged and neutral plasma particles pass from the target to the substrate; in this case, the electrons are to be absorbed by the electrodes using two electrodes, which are arranged parallel to and opposite one another and are oriented orthogonally to the target surface, through the application of a voltage of a level of 1000 V.
Therefore, it is an object of the invention to reduce the proportion of droplets and particles in a layer which is applied to a substrate using a PVD method, while at the same time allowing relatively large-area coating.
This objective preferably is achieved by the characterizing features of the inventive devices and methods for coating substrates in a vacuum. Advantageous embodiments and refinements of the invention will be apparent from the description provided herein.
According to the invention, the procedure is that an additional absorber electrode, which is at an electrically positive potential with respect to the plasma, is used, and an electric field is generated around this absorber electrode. Ionized particles and electrons of a plasma are passed through this electric field, with the result that electrically negative particles are absorbed by the absorber electrode and positive particles, preferably with a low mass/charge ratio of the plasma, reach the substrate. Neutral particles and particles with a high mass/charge ratio are only insignificantly affected in terms of their reference size and can therefore be separated.
Compared to the known magnetic filter systems which are in use, an absorber electrode of this type is significantly easier and less expensive to produce and operate.
A particularly advantageous embodiment of the invention consists in the invention being used in a device in which a pulsed vacuum arc discharge with a pulsed laser beam which is directed at the surface of a target connected as cathode is used. Such devices together with the corresponding methods are described, for example, in DE 39 01 401 C2 and, in improved form, in DD 279 695 B5. The vacuum arc discharge for generating a plasma is ignited between the target and an anode, and the ionized particles of the plasma are then deposited on a substrate in the form of a layer. A very wide variety of shapes can be used for the form of the anode and the target employed, and a very wide range of target materials can be employed. In methods as described, for example, in DD 279 695 B5, the drawbacks of the relatively large particles being deposited in the layer which are known from the prior art still occur. To counteract this, according to the invention an additional absorber electrode is used, which is held at an electrically more positive potential than the anode which may in any case already be present. With this arrangement, it is possible to separate the differently charged particles of the ionized particle flow out of the plasma, the electrically negative particles being at least for the most part absorbed by the absorber electrode and, in the form which is desired according to the invention, only the electrically positive particles of the particle flow passing through an electric field generated around an absorber electrode and onto the substrate. In this context, it is particularly beneficial for the distance between the anode and the absorber electrode to be relatively short, for example to be limited to about a few mm.
However, the invention may also be employed in a device or a method in which the plasma is generated exclusively by means of an arc discharge in a vacuum, as described, inter alia, in DD 280 338 B5. In this case, the arc discharge is initiated solely by an increase in voltage or in conjunction with the generation of a short circuit. In this case too, a target connected as cathode and an anode in a vacuum chamber are used once again, between which an arc is ignited, preferably in pulsed form, and a plasma is generated from the target material. However, if, after ignition, a continuous arc discharge is used, the arc should be guided along the surface of the target with the aid of controllable magnetic fields.
In this case too, the absorber electrode is once again arranged in the immediate vicinity of the target and/or the anode. An electric field is generated around the absorber electrode, the absorber electrode once again being held at an electrically positive potential compared to the plasma, so that the positive particles of the particle flow are moved out of the plasma toward the substrate, where the layer is formed, without the larger particles or particles with a higher mass charge in the particle flow reaching the substrate, since they come into contact with the absorber electrode, where they can be absorbed.
However, the absorber electrode which is to be used according to the invention may also be used in a device or a method in which the plasma is generated from a target using a laser beam which is directed onto this target. A procedure of this type is described, for example, in U.S. Pat. No. 4,987,007. In this case, it is also possible to generate a plasma from a target material which is not conductive. In this case too; the absorber electrode is once again arranged in the immediate vicinity of the root of the plasma, i.e. the point where the laser beam is focused onto the target. In this case too, the separation of the differently charged particles of the ionized particle flow is carried out in the form which has already been described for the other options, so that virtually only positively charged particles of the particle flow reach the substrate, where they form the layer.
The invention may advantageously be developed further as a result of the absorber electrode and/or the substrate being arranged or formed in such a way that it is impossible for any particles and ions from the plasma to reach the substrate directly. For this purpose, it is also additionally possible to employ a shielding diaphragm which may be arranged accordingly between target and substrate.
An element which forms the absorber electrode should encompass a plane which is arranged, oriented and dimensioned in such a way that the particle flow of the plasma cannot reach the substrate directly. In the most simple case, it may be a correspondingly inclined planar plane. However, it is also possible for a convexly curved plane to be encompassed.
A plane of this type does not necessarily have to be a continuous area, but rather it is also possible to use elements with gaps, such as for example grids, perforated or slotted metal sheets or other structural elements.
The inventive effect of the absorber electrode used is based on the high degree of ionization of the plasma which can be achieved with the methods listed above without the ions having to have a high kinetic energy. The energy of the ions is in this case between 30 and 100 eV. It is known that the degree of ionization of a plasma in the case of a vacuum arc discharge is approximately 80 to 90%.
The absorber electrode has the sole function of separating the differently charged particles of the particle stream out of the plasma and does not cause the ionized particles to be accelerated.
The arrangement of the absorber electrode in the immediate vicinity of the anode or target leads to a very large proportion of the electrons and negatively charged ions being absorbed and, at the same time, prevents the degree of ionization of the ions from changing significantly, through recombination processes, before leaving the diverter arrangement which is formed by the absorber electrode together with the electric field.
The coating rate can be positively influenced if the absorber electrode, the substrate and the other components which may be required for the generation of the plasma are designed and/or arranged in an advantageous way. Particularly suitable embodiments are described in more detail below.
With a corresponding shape of the absorber electrode, it is possible to ensure that, when the plasma enters the electric field generated around the absorber electrode, the electric field vector is oriented orthogonally to the direction of movement of the ion current, so that there is only a slight influence on the kinetic energy of the ions. For this reason, virtually only the ions can be optimally diverted as positively charge space charge to the substrate, on account of the effect of the electric field. The substrate is only slightly heated, so that the actual coating operation can be carried out virtually at room temperature, so that even corresponding thermally sensitive substrates can be coated without difficulty.
The energy of the ions and therefore also the properties of the layer formed can be influenced in a controlled way by a negative voltage applied to the substrate.
Furthermore, it may be beneficial for an element which is in grid form, for example, and is made from an electrically conductive material to be arranged between the root of the plasma and the absorber electrode, through which element the plasma is passed. A grid-like element of this type may be at the electric potential of the anode. Moreover, it may be advantageous for the grid-like element to be designed so as to be curved toward the direction of movement of the plasma. The grid-like element may be connected to the anode and, if appropriate, a diaphragm which is used and consequently may also be attached thereto.
The invention may advantageously also be used to form reactively influenced layers. Gases can be supplied for this purpose. These gases are ionized in the vicinity of the absorber electrode and are chemically activated, so that, for example, oxide, carbide or nitride layers or a combination, such as for example carbonitrides, can be produced using, for example, nitrogen, oxygen, H2, hydrocarbons, at a low mass flow rate and consequently very low gas pressures of below 10xe2x88x921 Pa.